As diverse and data-heavy underwater applications emerge, demanding requirements are further imposed on underwater wireless communication systems. Future underwater wireless communication networks might consist of both mobile and stationary nodes which exchange data such as control, telemetry, speech, and video signals among themselves as well as a central node located at a ship or onshore. The submerged nodes, which can, for example, take the form of an autonomous underwater vehicle/robot or diver, can be equipped with various sensors, sonars, video cameras, or other types of data acquisition instruments. Innovative physical layer solutions are therefore required to develop efficient, reliable, and high-speed transmission solutions tailored for challenging and diverse requirements of underwater applications. Building on the promising combination of multi-carrier and cooperative communication techniques, this dissertation investigates the fundamental performance bounds of cooperative underwater acoustic (UWA) communication systems taking into account the inherent unique characteristics of the UWA channel. We derive outage probability and capacity expressions for cooperative multi-carrier UWA systems with amplify-and-forward and decode-and-forward relaying. Through the derived expressions, we demonstrate the effect of several system and channel parameters on the performance. Furthermore, we investigate the performance of cooperative UWA systems in the presence of non-uniform Doppler distortion and propose receiver designs to mitigate the degrading Doppler effects.

Smart phones and tablet computers have greatly boosted the demand for services via wireless access points, keeping constant pressure on the network providers to deliver vast amounts of data over the wireless infrastructure. To enlarge coverage and enhance throughput, relaying has been adopted in the new generation of wireless communication systems, such as in the Long-Term Evolution Advanced standard,  and will continue to play an important role in the next generation wireless infrastructure. Depending on functionality, relaying can be characterizing into three main categories: amplify-and-forward (AF), compression-and-forward (CF), and decode-and-forward (DF).  In this thesis, we investigate different cooperative strategies in wireless networks when relaying is in use. We first investigate  the capacity outer and inner bounds for a wireless multicast relay network where two sources, connected by error-free backhaul, multicast to two destinations with the help of a full-duplex relay node.  For high-rate backhaul scenarios, we find the exact cut-set bound of the capacity region by extending the proof of the converse for the Gaussian relay channel. For low-rate backhaul scenarios, we present two genie-aided outer bounds by extending the previous proof and introducing two lemmas on conditional (co-)variance. Our inner bounds are derived from various cooperative strategies by combining DF/CF/AF relaying with network coding schemes. We also extend the noisy network coding scheme and the short-message noisy network coding approach to correlated sources. For low-rate backhaul, we propose a new coding scheme, partial-decode-and-forward based linear network coding. We derive the achievable rate regions  for these schemes and measure the performance in term of achievable rates over Gaussian channels. By numerical investigation we observe significant gains over benchmark schemes and demonstrate that the gap between upper and lower bounds is in general not large. We also show that for high-rate backhaul, the cut-set bound can be achieved  when the signal-to-noise ratios lie in the sphere defined by the source-relay and relay-destination channel gains. For wireless networks with independent noise, we propose a simple framework to get capacity outer and inner bounds based on the ``one-shot'' bounding models. We first extend the models for two-user broadcast channels to many-user scenarios and then establish the gap between upper and lower bounding models. For networks with coupled links, we propose  a channel decoupling method which can decompose the network into overlapping multiple-access channels and broadcast channels.  We then apply the one-shot models and create an upper bounding network with only  bit-pipe connections. When developing the lower bounding network, we propose a  two-step update of these models for each coupled broadcast and multiple-access channels. We demonstrate by some examples that the resulting upper bound is in general very good and the gap between the upper and lower bounds is usually not large. For relay-aided downlink scenarios, we propose a cooperation scheme by cancelling interference at the transmitter. It is indeed a symbol-by-symbol approach to one-dimension dirty paper coding (DPC). For finite-alphabet signaling and interference, we derive the optimal (in terms of maximum mutual information) modulator under a given power constraint. A sub-optimal modulator is also proposed by formulating an optimization problem that maximizes the minimum distance of the signal constellation, and this non-convex optimization problem is approximately solved by semi-definite relaxation.  Bit-level simulation shows that the optimal and sub-optimal modulators can achieve significant gains over the Tomlinson-Harashima precoder (THP) benchmark and over non-DPC reference schemes, especially when the power of the interference is larger than the power of the noise. / <p>QC 20121015</p>

In order to improve the Quality of Service in wireless networks it is crucial to design and optimize the communication algorithms based on the underlying Physical and Link Layers. In this thesis we show that if instead of the link abstraction used in traditional wireless networking we rely on the much broader definition of a link, used in the context of cooperative communication, we can improve the performance of relay transmission systems operating over the wireless medium. From a networking perspective there are a whole host of layering and cross-layer design issues that enable one to propose optimal cooperative algorithms for wireless communication. Most of the research in this area has been concentrated on the physical layer issues. In this thesis, we consider the interaction of the physical layer cooperative link with the higher layers, in particular the Medium Access Control Layer, and show that by appropriate protocol design we can improve the performance of wireless networks by using cooperation. Enabling cooperation among nodes in an optimal manner can lead to significant increase in the throughput for multi-hop wireless networks. We study and design cooperative protocols that lead to this throughput increase and quantify the appropriate level of cooperation among the users which leads to improving QoS.

In this thesis, we argue for node selection in cooperative decode-and-forward networks. In a single-hop network with multiple relays, we show that selecting a single node to aid in the transmission between a source and a destination outperforms both traditional orthogonal transmissions and distributed space-time codes. In networks where sources transmit information over multiple hops and relays can communicate with each other, we study the relationship between cooperation and channel-adaptive routing. We show that cooperation is only beneficial if designed jointly with a routing scheme. This motivates a search for optimal algorithms in generalized relay networks. In networks without restrictions on the relays in terms of whom they can communicate with, we study the problem of optimal resource allocation in terms of transmission time. The resource allocation selects the relays to participate in the transmission and optimally allocates time resource between the selected relays. To implement this resource allocation algorithm, we propose a recursive solution which reduces the computational complexity of the algorithm. For large networks, the resulting computational complexity of implementing the algorithm is exponential in the size of the network and is likely to preclude its implementation. We thus propose that the resource allocation be implemented sub-optimally through node selection: a subset of the nodes in the network should be selected and used as input to the optimal resource allocation algorithm. We provide guidelines for selecting the nodes and propose four heuristics which offer various complexity-performance trade-offs. Compared to the optimal resource algorithm, all four heuristics significantly decrease the required computation complexity of the optimal algorithm.

We consider a system with a single base station communicating with multiple users over orthogonal channels while being assisted by multiple relays. Several recent works have suggested that, in such a scenario, selection, i.e., a single relay helping the source, is the best relaying option in terms of the resulting complexity and overhead. However, in a multiuser setting, optimal relay assignment is a combinatorial problem. We formulate a related convex optimization problem that provides an extremely tight upper bound on performance and show that selection is, almost always, inherent in the solution. We also provide a heuristic to find a close-to-optimal relay assignment and power allocation across users supported by a single relay. Simulation results using realistic channel models demonstrate the efficacy of the proposed schemes, but also raise the question as to whether the gains from relaying are worth the additional costs.

Vehicular Ad hoc Networks (VANET) consist of radio-equipped vehicles and roadside units (RSU) and support many safety and commercial applications. Multi-hop forwarding can extend the communication range of both RSUs and vehicular broadcasts. Recently, the use of Positive Orthogonal Codes (POC) as transmission patterns of repetition-based broadcast medium access control (MAC) for safety messages has been proposed. This thesis proposes a cooperative forwarding protocol in which multiple relays at each forwarding hop form a virtual relay and coordinate their transmission times to correspond to a POC codeword. The protocol thereby exploits spatial diversity while conforming to the POC-based MAC, resulting in fewer collisions and mitigating the effect of hidden terminals. The design is validated through NS2 simulations, which show comparable performance with other forwarding schemes while producing significantly less performance degradation for safety message broadcasts on the same channel.

Vehicular Ad hoc Networks (VANET) consist of radio-equipped vehicles and roadside units (RSU) and support many safety and commercial applications. Multi-hop forwarding can extend the communication range of both RSUs and vehicular broadcasts. Recently, the use of Positive Orthogonal Codes (POC) as transmission patterns of repetition-based broadcast medium access control (MAC) for safety messages has been proposed. This thesis proposes a cooperative forwarding protocol in which multiple relays at each forwarding hop form a virtual relay and coordinate their transmission times to correspond to a POC codeword. The protocol thereby exploits spatial diversity while conforming to the POC-based MAC, resulting in fewer collisions and mitigating the effect of hidden terminals. The design is validated through NS2 simulations, which show comparable performance with other forwarding schemes while producing significantly less performance degradation for safety message broadcasts on the same channel.

We consider a system with a single base station communicating with multiple users over orthogonal channels while being assisted by multiple relays. Several recent works have suggested that, in such a scenario, selection, i.e., a single relay helping the source, is the best relaying option in terms of the resulting complexity and overhead. However, in a multiuser setting, optimal relay assignment is a combinatorial problem. We formulate a related convex optimization problem that provides an extremely tight upper bound on performance and show that selection is, almost always, inherent in the solution. We also provide a heuristic to find a close-to-optimal relay assignment and power allocation across users supported by a single relay. Simulation results using realistic channel models demonstrate the efficacy of the proposed schemes, but also raise the question as to whether the gains from relaying are worth the additional costs.

In order to improve the Quality of Service in wireless networks it is crucial to design and optimize the communication algorithms based on the underlying Physical and Link Layers. In this thesis we show that if instead of the link abstraction used in traditional wireless networking we rely on the much broader definition of a link, used in the context of cooperative communication, we can improve the performance of relay transmission systems operating over the wireless medium. From a networking perspective there are a whole host of layering and cross-layer design issues that enable one to propose optimal cooperative algorithms for wireless communication. Most of the research in this area has been concentrated on the physical layer issues. In this thesis, we consider the interaction of the physical layer cooperative link with the higher layers, in particular the Medium Access Control Layer, and show that by appropriate protocol design we can improve the performance of wireless networks by using cooperation. Enabling cooperation among nodes in an optimal manner can lead to significant increase in the throughput for multi-hop wireless networks. We study and design cooperative protocols that lead to this throughput increase and quantify the appropriate level of cooperation among the users which leads to improving QoS.

In this thesis, we argue for node selection in cooperative decode-and-forward networks. In a single-hop network with multiple relays, we show that selecting a single node to aid in the transmission between a source and a destination outperforms both traditional orthogonal transmissions and distributed space-time codes. In networks where sources transmit information over multiple hops and relays can communicate with each other, we study the relationship between cooperation and channel-adaptive routing. We show that cooperation is only beneficial if designed jointly with a routing scheme. This motivates a search for optimal algorithms in generalized relay networks. In networks without restrictions on the relays in terms of whom they can communicate with, we study the problem of optimal resource allocation in terms of transmission time. The resource allocation selects the relays to participate in the transmission and optimally allocates time resource between the selected relays. To implement this resource allocation algorithm, we propose a recursive solution which reduces the computational complexity of the algorithm. For large networks, the resulting computational complexity of implementing the algorithm is exponential in the size of the network and is likely to preclude its implementation. We thus propose that the resource allocation be implemented sub-optimally through node selection: a subset of the nodes in the network should be selected and used as input to the optimal resource allocation algorithm. We provide guidelines for selecting the nodes and propose four heuristics which offer various complexity-performance trade-offs. Compared to the optimal resource algorithm, all four heuristics significantly decrease the required computation complexity of the optimal algorithm.